Semiconductor laser

ABSTRACT

A semiconductor laser includes a semiconductor substrate of a first conductivity type and having a front surface; a first semiconductor layer disposed on the front surface of the semiconductor substrate and having a refractive index that increases with distance from the semiconductor substrate; an active layer disposed on the first semiconductor layer; and a second semiconductor layer disposed on the active layer, having a refractive index that decreases with distance from the active layer, and having a ridge. In this laser, the refractive index distribution between the ridge and the substrate is asymmetrical about the active layer so that the center of the light intensity distribution shifts from the active layer toward the substrate, in the direction perpendicular to the front surface of the substrate. Therefore, propagated light is hardly affected by the refractive index distribution in the width direction of the laser, which is caused by the presence of the ridge, whereby occurrence of a higher mode is suppressed.

FIELD OF THE INVENTION

[0001] The present invention relates to semiconductor lasers and, moreparticularly, to a semiconductor laser used as a light source forinformation processing or optical communication.

BACKGROUND OF THE INVENTION

[0002]FIG. 9(a) is a cross-sectional view of a ridge type semiconductorlaser disclosed by J. Hashimoto et.al. in IEEE Journal of QuantumElectron, Vol.33, pp.66-77, 1997. This laser comprises a p sideelectrode 1, an insulating film 2, a p type GaAs contact layer 3, a ptype GaInP upper cladding layer 4 having a stripe-shaped ridge extendingin the resonator length direction, a ridge side undoped GaInAsP secondguide layer 5, a ridge side undoped GaAs first guide layer 6, an activelayer 7, a substrate side undoped GaAs first guide layer 8 having thesame composition ratio and thickness as those of the ridge side firstguide layer 6, a substrate side undoped GaInAsP second guide layer 9having the same composition ratio and thickness as those of the ridgeside second guide layer 5, an n type GaInP lower cladding layer 10having the same composition ratio as that of the upper cladding layer 4and the same thickness as that of the ridge of the cladding layer 4, ann type GaAs buffer layer 11, an n type GaAs substrate 12, and an n sideelectrode 13. FIG. 9(b) is a graph showing the refractive index profileof the semiconductor laser in the direction perpendicular to the surfaceof the substrate 12. In FIG. 9(a), “z” shows the resonator lengthdirection, “x” shows the direction perpendicular to the surface of thesubstrate 12 (hereinafter referred to as “thickness direction”), and “y”shows the direction perpendicular to both of the resonator lengthdirection z and the thickness direction x (hereinafter referred to as“width direction”).

[0003] A description is given of the operation of the semiconductorlaser. Holes and electrons are injected through the upper cladding layer4 and the lower cladding layer 10 into the active layer 10,respectively, and recombine to generate light. The light so generated ispropagated along the resonator length direction z while being influencedby the refractive indices in the thickness direction x and the widthdirection y, and it is amplified while being reflected between thefacets of the laser, resulting in laser oscillation.

[0004] In this prior art semiconductor laser, the refractive indexdistribution in the thickness direction x is symmetrical about theactive layer 7, until reaching the upper cladding layer 4 and the lowercladding layer 10 which are disposed on and beneath the active layer 7,respectively. That is, as shown in FIG. 9(b), the ridge side first guidelayer 6, the ridge side second guide layer 5, and the upper claddinglayer 4 have the same refractive indices and the same thicknesses asthose of the substrate side first guide layer 8, the substrate sidesecond guide layer 9, and the lower cladding layer 10, respectively.

[0005] As described above, in the prior art ridge type semiconductorlaser, since the refractive index distribution in the thicknessdirection x is symmetrical about the active layer 7, the propagatedlight is distributed almost symmetrically about the active layer 7 inthe thickness direction x. However, since a refractive index differenceis generated in the width direction y because of the ridge of the uppercladding layer 4, when the propagated light is distributed almostsymmetrically about the active layer 7 as described above, the influenceof the refractive index in the width direction y on the propagated lightat the ridge becomes significant, whereby a higher mode of oscillationis permitted. As a result, kinks occur due to mode competition duringlow-power output operation, and the output power cannot be increased inthe practical use.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a ridge typesemiconductor laser that can shift the level at which a higher modeoccurs, toward the higher power level.

[0007] Other objects and advantages of the invention will becomeapparent from the detailed description that follows. The detaileddescription and specific embodiments described are provided only forillustration since various additions and modifications within the scopeof the invention will be apparent to those of skill in the art from thedetailed description.

[0008] According to a first aspect of the present invention, there isprovided a semiconductor laser including a semiconductor substrate of afirst conductivity type and having a front surface; a firstsemiconductor layer disposed on the front surface of the semiconductorsubstrate and having a refractive index that increases with distancefrom the semiconductor substrate; an active layer disposed on the firstsemiconductor layer; and a second semiconductor layer disposed on theactive layer, having a refractive index that decreases with distancefrom the active layer, and having a ridge; wherein the refractive indexdistribution between the ridge and the substrate is asymmetrical aboutthe active layer so that the center of the light intensity distributionshifts from the active layer toward the substrate, in the directionperpendicular to the front surface of the substrate. Therefore, morelight is distributed to the substrate side than the ridge side, and theinfluence of the refractive index distribution in the width direction onpropagated light is reduced, which distribution occurs due to adifference in refractive indices between the ridge of the secondsemiconductor layer and portions of the layer outside the ridge, therebyavoiding occurrence of a higher mode that causes kinks. As a result, thelight output level at which kinks occur is increased, providing a ridgetype semiconductor laser capable of high-power output operation in thepractical use.

[0009] According to a second aspect of the present invention, in theabove-mentioned semiconductor laser, the first semiconductor layercomprises a lower cladding layer of the first conductivity type andhaving a refractive index, and a substrate side guide layer disposed onthe lower cladding layer and having a thickness and a refractive index;the second semiconductor layer comprises a ridge side guide layer havinga thickness and a refractive index, and an upper cladding layer of asecond conductivity type, opposite the first conductivity type, disposedon the ridge side guide layer and having a refractive index; and atleast one of the thickness and the refractive index of the substrateside guide layer is larger than that of the ridge side guide layer.Therefore, more light is distributed to the substrate side, and thelight output level at which kinks occur is increased, resulting in aridge type semiconductor laser capable of high-power output operation inthe practical use.

[0010] According to a third aspect of the present invention, in theabove-mentioned semiconductor laser, the refractive index of the lowercladding layer is larger than that of the upper cladding layer.Therefore, more light is distributed to the substrate side, and thelight output level at which kinks occur is increased, resulting in aridge type semiconductor laser capable of high-power output operation inthe practical use. In addition, the expansion of the far field patternis reduced, whereby the aspect ratio of light is reduced.

[0011] According to a fourth aspect of the present invention, in theabove-mentioned semiconductor laser, the first semiconductor layerincludes a lower cladding layer of the first conductivity type and has athickness and a refractive index; the second semiconductor layerincludes an upper cladding layer of a second conductivity type, oppositethe first conductivity type, and has a thickness and a refractive index;and at least one of the thickness and the refractive index of the lowercladding layer is larger than that of the upper cladding layer.Therefore, more light is distributed to the substrate side, and thelight output level at which kinks occur is increased, resulting in aridge type semiconductor laser capable of high-power output operation inthe practical use. Especially when the refractive index of the lowercladding layer is increased, the expansion of the far field pattern isreduced, whereby the aspect ratio of light is reduced.

[0012] According to a fifth aspect of the present invention, in theabove-mentioned semiconductor laser, the refractive index of the lowercladding layer continuously increases toward the active layer; and therefractive index of the upper cladding layer continuously decreases withdistance from the active layer. Therefore, more light is distributed tothe substrate side, and the light output level at which kinks occur isincreased, resulting in a ridge type semiconductor laser capable ofhigh-power output operation in the practical use.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGS. 1(a) and 1(b) are diagrams for explaining a semiconductorlaser according to a first embodiment of the present invention.

[0014] FIGS. 2(a) and 2(b) are diagrams for explaining a semiconductorlaser according to a second embodiment of the present invention.

[0015]FIG. 3 is a diagram showing simulation results of thesemiconductor laser according to the second embodiment.

[0016] FIGS. 4(a) and 4(b) are diagrams for explaining a semiconductorlaser according to a third embodiment of the present invention.

[0017] FIGS. 5(a) and 5(b) are diagrams for explaining a semiconductorlaser according to a fourth embodiment of the present invention.

[0018] FIGS. 6(a) and 6(b) are diagrams for explaining a semiconductorlaser according to a fifth embodiment of the present invention.

[0019]FIG. 7 is a diagram showing simulation results of thesemiconductor laser according to the fifth embodiment.

[0020] FIGS. 8(a) and 8(b) are diagrams for explaining a semiconductorlaser according to a sixth embodiment of the present invention.

[0021] FIGS. 9(a) and 9(b) are diagrams for explaining a semiconductorlaser according to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] [Embodiment 1]

[0023]FIG. 1(a) is a cross-sectional view illustrating a semiconductorlaser according to a first embodiment of the present invention, and FIG.1(b) is a graph showing the refractive index profile of the laser in thethickness direction x. This laser includes an n type GaAs substrate 12.An n type AlGaAs lower cladding layer 10 a (first semiconductor layer)is disposed on the substrate 12. An undoped InGaAs active layer 7 a isdisposed on the lower cladding layer 10 a. A p type AlGaAs uppercladding layer 4 a (second semiconductor layer) is disposed on theactive layer 7 a. The upper cladding layer 4 a has a stripe-shaped ridgeextending in the resonator length direction z, at its surface. A p typeGaAs contact layer 3 is disposed on the ridge of the upper claddinglayer 4 a. The surface of the upper cladding layer 4 a including theridge is covered with an insulating film 2, such as a silicon oxidefilm, except the top of the contact layer 3. A p side electrode 1 isdisposed on the insulating film 2 and on the contact layer 3. An n sideelectrode 13 is disposed on the rear surface of the substrate 12.

[0024] The thickness of the substrate side lower cladding layer 10 a islarger than the thickness of the ridge side upper cladding layer 4 a,and the refractive index of the lower cladding layer 10 a at a portioncontacting the substrate is larger than the refractive index of theupper cladding layer 4 a at the top of the ridge. In this firstembodiment, the difference in refractive indices between the lowercladding layer 10 a and the upper cladding layer 4 a is obtained bysetting the Al composition ratio of the lower cladding layer 10 asmaller than the Al composition ratio of the upper cladding layer 4 a.In each of the cladding layers, the refractive index gradually decreaseswith distance from the active layer 7 a. Thereby, the refractive indexdistribution between the upper cladding layer 4 a and the lower claddinglayer 10 a in the thickness direction x is asymmetrical about the activelayer 7 a.

[0025] A description is given of the method of fabricating thesemiconductor laser. Initially, the lower cladding layer 10 a, theactive layer 7 a, the upper cladding layer 4 a, and the contact layer 3are successively grown on the surface of the substrate 12. The lowercladding layer 10 a is grown so that the Al composition ratiocontinuously decreases, and the upper cladding layer 4 a is grown sothat the Al composition ratio continuously increases.

[0026] Next, a stripe-shaped mask comprising an insulating film (notshown) is formed on the contact layer 3 and, using this mask, thecontact layer 3 and an upper portion of the upper cladding layer 4 a areselectively etched to form a ridge. After removal of the mask, theinsulating film 2 is formed over the surface of the structure, and aportion of the insulating film 2 at the top of the contact layer 3 isremoved by etching. Further, the p side electrode 1 is formed on theinsulating film 2, contacting the contact layer 3, and the n sideelectrode 13 is formed on the rear surface of the substrate 12. Finally,resonator facets are formed by cleaving, thereby completing thesemiconductor laser shown in FIG. 1.

[0027] A description is given of the operation of the laser. Holes andelectrons are injected into the active layer 7 a through the ridge ofthe upper cladding layer 4 a and the lower cladding layer 10 a,respectively, and recombine to generate light. The light so generated istransmitted in the resonator length direction z while being influencedby the refractive indices in the thickness direction x and the widthdirection y, and it is amplified while being reflected between thefacets, resulting in laser oscillation.

[0028] In the semiconductor laser according to this first embodiment, asdescribed above, the thickness and refractive index of the lowercladding layer 10 a on the substrate side are larger than those of theupper cladding layer 4 a on the ridge side so that the refractive indexdistribution in the thickness direction x becomes asymmetrical about theactive layer 7 a. Since the refractive index distribution in thethickness direction x is so set, more light is distributed to thesubstrate side than the ridge side when viewed from the active layer 7a. Therefore, the influence of the refractive index distribution in thewidth direction y on the propagated light is reduced, which distributionoccurs due to a difference in refractive indices between the ridge ofthe upper cladding layer 4 a and portions of the layer 4 a outside theridge, thereby avoiding occurrence of a higher mode that causes kinks.As a result, practical high-power operation is realized. In addition,only the fundamental mode is permitted even when the ridge width isincreased.

[0029] In this first embodiment of the invention, the thickness andrefractive index of the lower cladding layer 10 a are set larger thanthose of the upper cladding layer 4 a to make the refractive indexdistribution between the lower cladding layer 10 a and the uppercladding layer 4 a asymmetrical about the active layer 7 a. However, thepresent invention is not restricted thereto, and any structure may beadopted as long as the refractive index distribution becomesasymmetrical so that the center (peak) of the light intensitydistribution shifts from the active layer 7 a toward the substrate 12.For example, one of the thickness and the refractive index of the lowercladding layer 10 a may be set larger than that of the upper claddinglayer 4 a to obtain the asymmetric refractive index distribution.

[0030] Furthermore, although in this first embodiment the refractiveindices of the upper and lower cladding layers 4 a and 10 a continuouslyincrease toward the active layer 7 a, the refractive indices of therespective cladding layers may be fixed in the thickness direction x aslong as the thickness and refractive index of the lower cladding layerare larger than those of the upper cladding layer.

[0031] [Embodiment 2]

[0032]FIG. 2(a) is a cross-sectional view illustrating a semiconductorlaser according to a second embodiment of the invention, and FIG. 2(b)is a graph showing the refractive index profile of the laser in thethickness direction x. The semiconductor laser according to this secondembodiment is different from the semiconductor laser according to thefirst embodiment in the following points. While in the first embodimentthe lower cladding layer 10 a (first semiconductor layer) comprising asingle semiconductor layer is disposed between the active layer 7 a andthe substrate 12, in this second embodiment a first compositesemiconductor layer is disposed between the active layer 7 a and thesubstrate 12, which composite layer comprises an n type AlGaAs lowercladding layer 10 b, a substrate side second guide layer 9 a comprisingundoped AlGaAs and having a thickness g_(L2), and a substrate side firstguide layer 8 a comprising undoped-GaAs and having a thickness g_(L1).Further, in place of the upper cladding layer 4 a (second semiconductorlayer) on the active layer 7 a in the first embodiment, a secondcomposite semiconductor layer is disposed on the active layer 7 a inthis second embodiment, which composite layer comprises a ridge sidefirst guide layer 6 a comprising undoped GaAs and having a thicknessg_(U1), a ridge side second guide layer 5 a comprising undoped AlGaAsand having a thickness g_(U2), and a p type AlGaAs upper cladding layer4 b having a ridge at its surface. The thickness g_(L1) of the substrateside first guide layer 8 a is larger than the thickness g_(U1) of theridge side first guide layer 6 a, and the thickness gL2 of the substrateside second guide layer 9 a is larger than the thickness g_(U2) of theridge side second guide layer 5 a. The lower cladding layer 10 b, thesubstrate side first guide layer 8 a, the substrate side second guidelayer 9 a, the ridge side first guide layer 6 a, the ridge side secondguide layer 5 a, and the upper cladding layer 4 b respectively havefixed refractive indices in the thickness direction x. The Alcomposition ratio of the substrate side second guide layer 9 a issmaller than the Al composition ratio of the lower cladding layer 10 b,and the Al composition ratio of the ridge side second guide layer 5 a islarger than the Al composition of the upper cladding layer 4 b. As aresult, in the first composite semiconductor layer, the refractive indexincreases stepwise with distance from the substrate 12 and, in thesecond composite semiconductor layer, the refractive index decreasesstepwise with distance from the active layer 7 a. In FIGS. 2(a) and2(b), the same reference numerals as those in FIGS. 1(a) and 1(b)designate the same or corresponding parts. In FIG. 2(a), W denotes theridge width, and t denotes the thickness of a portion of the uppercladding layer at each side of the ridge, i.e., a portion outside theridge.

[0033] The fabrication method of the semiconductor laser according tothis second embodiment is identical to the fabrication method alreadydescribed for the first embodiment except that the lower cladding layer10 b, the second guide layer 9 a, and the first guide layer 8 a aregrown instead of the lower cladding layer 10 a, and the first guidelayer 6 a, the second guide layer 5 a, and the upper cladding layer 4 bare grown instead of the upper cladding layer 4 a.

[0034] In the semiconductor laser according to this second embodiment,as shown in FIG. 2(b), the thickness g_(Ll) of the substrate side firstguide layer 8 a and the thickness gL2 of the substrate side second guidelayer 9 a are larger than the thickness g_(U1) of the ridge side firstguide layer 6 a and the thickness g_(U2) of the ridge side second guidelayer 5 a, respectively, so that the refractive index distributionbetween the upper cladding layer 4 b and the lower cladding layer 10 bin the thickness direction x is asymmetrical about the active layer 7 a.As the result of the asymmetrical distribution of refractive index, thecenter of the light intensity distribution of the propagated lightshifts from the active layer 7 a toward the substrate, so that morelight is distributed to the substrate side. Therefore, the propagatedlight is hardly affected by the refractive index distribution in thewidth direction y due to the presence of the ridge, whereby occurrenceof a higher mode is suppressed.

[0035] Hereinafter, the effects provided by increasing the thicknessesof the substrate side guide layers will be described on the basis ofsimulation results.

[0036]FIG. 3 is a graph showing the simulation results of thesemiconductor laser according to the second embodiment. In FIG. 3, theordinate shows the thickness t of the upper cladding layer 4 b outsidethe ridge, and the abscissa shows the ridge width W. Each of thecontinuous line and the dotted line shows the boundary between a regionwhere only the fundamental mode (0 order) is permitted and a regionwhere the primary mode (primary order) is also permitted, which boundaryis calculated from the thickness t and the ridge width W using theequivalent refractive index method. The region where only thefundamental mode is permitted is under the boundary, and the regionwhere the primary mode is also permitted is above the boundary. In thissimulation, the Al composition ratio of the upper cladding layer 4 b is0.3, the thickness of the ridge is t+1.4 μm, and the Al compositionratio of the ridge side second guide layer 5 a is 0.2. The active layer7 a has a quantum well structure in which a 20 nm thick GaAs layer isput between two 8 nm thick InGaAs layers having an In composition ratioof 0.15. The Al composition ratio of the substrate side second guidelayer 9 a is 0.2, and the Al composition ratio and the thickness of thelower cladding layer 10 a are 0.3 and t+1.4 μm, respectively. Thesimulation result shown by the dotted line is obtained when the ridgeside first guide layer 6 a and the substrate side first guide layer 8 ahave the same thickness of 0.02 μm, and the ridge side second guidelayer Sa and the substrate side second guide layer 9 a have the samethickness of 0.04 μm, whereby the refractive index distribution issymmetrical about the active layer 7 a (symmetrical structure). Thesimulation result shown by the continuous line is obtained when theridge side second guide layer 5 a is 0.04 μm thick, the ridge side firstguide layer 6 a is 0.02 μm thick, the substrate side first guide layer 8a is 0.04 μm thick, and the substrate side second guide layer 9 a is0.06 μm thick, whereby the refractive index distribution is asymmetricalas shown in FIG. 2(b) (asymmetrical structure). It can be seen from FIG.3 that the boundary between the fundamental mode and the primary modeshifts upward in the asymmetrical structure than in the symmetricalstructure. For example, when t is 0.40 μm, in the symmetrical structure,the boundary between the region where only the fundamental mode ispermitted and the region where the primary mode is also permitted ispresent at the ridge width W of about 3.65 μm. On the other hand, in theasymmetrical structure, when t is 0.40 μm, the boundary is present atthe ridge width W of about 4.05 μm. In FIG. 3, in the region between thecontinuous line and the dotted line, kinks easily occur as both thefundamental mode and the primary mode are permitted in this region inthe symmetrical structure. However, in the asymmetrical structureaccording to the second embodiment, since only the fundamental mode ispermitted in this region, occurrence of kinks is suppressed.

[0037] While in this second embodiment the thicknesses of both of thefirst and second guide layers 8 a and 9 a on the substrate side areincreased to shift the light intensity distribution toward the substrateside, the same effects as described above are achieved even when thethickness of one of the first and second guide layers 8 a and 9 a isincreased.

[0038] Further, while in this second embodiment two guide layers aredisposed at each of the upper and lower sides of the active layer 7 a, asingle guide layer may be disposed at each side of the active layer 7 a.Even in this case, the same effects as described above are achieved byincreasing the thickness of the substrate side guide layer.

[0039] Furthermore, even when three or more guide layers are disposed ateach side of the active layer 7 a, the same effects as described aboveare achieved by increasing the thickness of at least one of thesubstrate side guide layers.

[0040] Moreover, while in this second embodiment the refractive indicesof the respective guide layers disposed on and beneath the active layer7 a are fixed in the thickness direction, these refractive indices maybe continuously increased toward the active layer 7 a. Also in thiscase, the same effects as described above are achieved by increasing thethickness of the substrate side guide layer.

[0041] [Embodiment 3]

[0042]FIG. 4(a) is a cross-sectional view illustrating a semiconductorlaser according to a third embodiment of the invention, and FIG. 4(b) isa graph showing the refractive index profile of the laser in thethickness direction x. The semiconductor laser according to this thirdembodiment is different from the semiconductor laser according to thefirst embodiment in the following points. While in the first embodimentthe lower cladding layer 10 a (first semiconductor layer) comprising asingle semiconductor layer is disposed between the active layer 7 a andthe substrate 12, in this third embodiment a first compositesemiconductor layer is disposed between the active layer 7 a and thesubstrate 12, which composite layer comprises an n type AlGaAs lowercladding layer 10 b, a substrate side second guide layer 9 b comprisingundoped AlGaAs and having a refractive index nLg2, and a substrate sidefirst guide layer 8 b comprising undoped GaAs and having a refractiveindex n_(Lg1). Further, in place of the upper cladding layer 4 a (secondsemiconductor layer) on the active layer 7 a in the first embodiment, asecond composite semiconductor layer is disposed on the active layer 7 ain this third embodiment, which composite layer comprises a ridge sidefirst guide layer 6 b comprising undoped GaAs and having a refractiveindex n_(Ug1), a ridge side second guide layer 5 b comprising undopedAlGaAs and having a refractive index n_(Ug2), and a p type AlGaAs uppercladding layer 4 b having a ridge at its surface. The refractive indexn_(Lg1) of the substrate side first guide layer 8 b is larger than therefractive index n_(Ug1) of the ridge side first guide layer 6 b, andthe refractive index n_(Lg2) of the substrate side second guide layer 9b is larger than the refractive index n_(Ug2) of the ridge side secondguide layer 5 b. To set the refractive indices of these semiconductorlayers as mentioned above, the Al composition ratios of the respectivelayers are controlled. The substrate side first guide layer 8 b, thesubstrate side second guide layer 9 b, the ridge side first guide layer6 b, and the ridge side second guide layer 5 b respectively have fixedrefractive indices in the thickness direction x. The Al compositionratio of the substrate side second guide layer 9 b is smaller than theAl composition ratio of the lower cladding layer 10 b, and the Alcomposition ratio of the ridge side second guide layer 5 b is largerthan the Al composition ratio of the upper cladding layer 4 b. As aresult, in the first composite semiconductor layer, the refractive indexincreases stepwise with distance from the substrate 12 and, in thesecond composite semiconductor layer, the refractive index decreasesstepwise with distance from the active layer 7 a. In FIGS. 4(a) and4(b), the same reference numerals as those shown in FIGS. 1(a) and 1(b)designate the same or corresponding parts.

[0043] The fabrication method of the semiconductor laser according tothis third embodiment is identical to the fabrication method alreadydescribed for the first embodiment except that the lower cladding layer10 b, the substrate side the second guide layer 9 b, and the substrateside first guide layer 8 b are grown instead of the lower cladding layer10 a, and the ridge side first guide layer 6 b, the ridge side secondguide layer 5 b, and the upper cladding layer 4 b are grown instead ofthe upper cladding layer 4 a.

[0044] In the semiconductor laser according to this third embodiment, asshown in FIG. 4(b), the refractive index n_(Lg1) of the substrate sidefirst guide layer 8 b and the refractive index n_(Lg2) of the substrateside second guide layer 9 b are larger than the refractive index n_(Ug1)of the ridge side first guide layer 6 b and the refractive index n_(Ug2)of the ridge side second guide layer 5 b, respectively, so that therefractive index distribution between the upper cladding layer 4 b andthe lower cladding layer 10 b in the thickness direction x isasymmetrical about the active layer 7 a. As the result of theasymmetrical distribution of refractive index, the light intensitydistribution of the propagated light shifts toward the substrate withrespect to the active layer 7 a, and more light is distributed to thesubstrate side. Therefore, the light is hardly affected by therefractive index distribution in the width direction y which is causedby the presence of the ridge, whereby occurrence of a higher mode issuppressed.

[0045] While in this third embodiment the refractive indices of both ofthe substrate side first and second guide layers 8 b and 9 b areincreased, the same effects as described above are achieved even wheneither of these refractive indices is increased.

[0046] Further, even when a single guide layer is disposed at each ofthe upper and lower sides of the active layer 7 a, the same effects asdescribed above are achieved by increasing the refractive index of thesubstrate side guide layer.

[0047] Furthermore, even when three or more guide layers are disposed ateach of the upper and lower sides of the active layer 7 a, the sameeffects as described above are achieved by increasing the refractiveindex of at least one of the substrate side (lower side) guide layers.

[0048] Moreover, while in this third embodiment the refractive indicesof the respective guide layers disposed on and beneath the active layer7 a are fixed in the thickness direction, these refractive indices maybe continuously increased toward the active layer 7 a. Also in thiscase, the same effects as described above are achieved by increasing therefractive indices of the substrate side guide layers as a whole.

[0049] [Embodiment 4]

[0050]FIG. 5(a) is a cross-sectional view illustrating a semiconductorlaser according to a fourth embodiment of the invention, and FIG. 5(b)is a graph showing the refractive index profile of the laser in thethickness direction x.

[0051] The semiconductor laser according to this fourth embodiment isdifferent from the semiconductor laser according to the first embodimentin the following points. While in the first embodiment the lowercladding layer 1 a (first semiconductor layer) comprising a singlesemiconductor layer is disposed between the active layer 7 a and thesubstrate 12, in this fourth embodiment a first composite semiconductorlayer is disposed between the active layer 7 a and the substrate 12,which composite layer comprises an n type AlGaAs lower cladding layer 10b, a substrate side second guide layer 9 c comprising undoped AlGaAs andhaving a thickness g_(L2) and a refractive index nLg2, and a substrateside first guide layer 8 c comprising undoped GaAs and having athickness g_(L1) and a refractive index nLgl. Further, in place of theupper cladding layer 4 a (second semiconductor layer) on the activelayer 7 a in the first embodiment, a second composite semiconductorlayer is disposed on the active layer 7 a in this third embodiment,which composite layer comprises a ridge side first guide layer 6 ccomprising undoped GaAs and having a thickness g_(U1) and a refractiveindex n_(Ug1), a ridge side second guide layer 5 c comprising undopedAlGaAs and having a thickness gu₂ and a refractive index n_(Ug2), and ap type AlGaAs upper cladding layer 4 b having a ridge at its surface.The thickness g_(Lg1) of the substrate side first guide layer 8 c islarger than the thickness g_(U2) of the ridge side first guide layer 6c, and the thickness g_(L2) of the substrate side second guide layer 9 cis larger than the thickness g_(U2) of the ridge side second guide layer5 c. The refractive index n_(Lg1) of the substrate side first guidelayer 8 c is larger than the refractive index n_(Ug1) of the ridge sidefirst guide layer 6 c, and the refractive index n_(Lg2) of the substrateside second guide layer 9 c is larger than the refractive index nUg2 ofthe ridge side second guide layer 5 c. The substrate side first guidelayer 8 c, the substrate side second guide layer 9 c, the ridge sidefirst guide layer 6 c, and the ridge side second guide layer 5 crespectively have fixed refractive indices in the thickness direction x.The Al composition ratio of the substrate side second guide layer 9 c issmaller than the Al composition ratio of the lower cladding layer 10 b,and the Al composition ratio of the ridge side second guide layer 5 c islarger than the Al composition ratio of the upper cladding layer 4 b. Asa result, in the first composite semiconductor layer, the refractiveindex increases stepwise with distance from the substrate 12 and, in thesecond composite semiconductor layer, the refractive index decreasesstepwise with distance from the active layer 7 a. In FIGS. 5(a) and5(b), the same reference numerals as those shown in FIGS. 1(a) and 1(b)designate the same or corresponding parts.

[0052] The fabrication method of the semiconductor laser according tothis fourth embodiment is identical to the fabrication method alreadydescribed for the first embodiment except that the lower cladding layer10 b, the substrate side the second guide layer 9 c, and the substrateside first guide layer 8 c are successively grown instead of the lowercladding layer 10 a, and the ridge side first guide layer 6 c, the ridgeside second guide layer 5 c, and the upper cladding layer 4 b are growninstead of the upper cladding layer 4 a.

[0053] In this semiconductor laser, as shown in FIG. 5(b), the thicknessg_(L1) of the substrate side first guide layer 8 c and the thicknessg_(L2) of the substrate side second guide layer 9 c are larger than thethickness g_(U1) of the ridge side first guide layer 6 c and thethickness g_(U2) of the ridge side second guide layer 5 c, respectively,and the refractive index n_(Lg1) of the substrate side first guide layer8 c and the refractive index n_(Lg2) of the substrate side second guidelayer 9 c are larger than the refractive index n_(Ug1) of the ridge sidefirst guide layer 6 c and the refractive index n_(Ug2) of the ridge sidesecond guide layer 5 c, respectively, whereby the refractive indexdistribution between the upper cladding layer 4 b and the lower claddinglayer 10 b in the thickness direction x is asymmetrical about the activelayer 7 a. Thereby, the center of the light intensity distribution ofpropagated light shifts from the active layer 7 a toward the substrate12, so that more light is distributed to the substrate side than theridge side. As a result, the propagated light is hardly affected by therefractive index distribution in the width direction y due to thepresence of the ridge, and the same effects as provided by the firstembodiment are achieved.

[0054] In this fourth embodiment of the invention, the thicknesses andrefractive indices of the substrate side first and second guide layers 8c and 9 c are set larger than those of the ridge side first and secondguide layers 6 c and 5 c, respectively. However, the thickness andrefractive index of either of the substrate side first and second guidelayers 8 c and 9 c may be larger than those of the ridge side guidelayer, with the same effects as described above.

[0055] Further, even when a single guide layer is disposed at each ofthe upper and lower sides of the active layer 7 a, the same effects asdescribed above are obtained by setting the thickness and refractiveindex of the substrate side guide layer larger than those of the ridgeside guide layer.

[0056] Furthermore, even when three or more guide layers are disposed ateach of the upper and lower sides of the active layer 7, the sameeffects as mentioned above are obtained by setting the thickness andrefractive index of at least one of the guide layers on the substrateside (lower side) larger than those of the ridge side (upper side) guidelayer.

[0057] Moreover, although in this fourth embodiment the refractiveindices of the respective guide layers disposed on and beneath theactive layer 7 a are fixed in the thickness direction x, theserefractive indices may be continuously increased toward the active layer7 a. Also in this case, the same effects as described above are obtainedwhen the refractive indices and thicknesses of the substrate side guidelayers are larger than those of the ridge side guide layers.

[0058] [Embodiment 5]

[0059]FIG. 6(a) is a cross-sectional view of a semiconductor laseraccording to a fifth embodiment of the present invention, and FIG. 6(b)is a graph showing the refractive index profile of the laser in thethickness direction x.

[0060] The semiconductor laser according to this fifth embodiment isdifferent from the semiconductor laser according to the first embodimentin the following points. While in the first embodiment the lowercladding layer 10 a (first semiconductor layer) comprising a singlesemiconductor layer is disposed between the active layer 7 a and thesubstrate 12, in this fifth embodiment a first composite semiconductorlayer is disposed between the active layer 7 a and the substrate 12,which composite layer comprises an n type AlGaAs lower cladding layer 10c having a refractive index n_(Lc), a substrate side second guide layer9 d comprising undoped AlGaAs and having a thickness g_(L2) and arefractive index n_(Lg2), and a substrate side first guide layer 8 dcomprising undoped GaAs and having a thickness g_(L1) and a refractiveindex n_(Lg1). Further, in place of the upper cladding layer 4 a (secondsemiconductor layer) on the active layer 7 a in the first embodiment, asecond composite semiconductor layer is disposed on the active layer 7 ain this fifth embodiment, which composite layer comprises a ridge sidefirst guide layer 6 d comprising undoped GaAs and having the samerefractive index and thickness as those of the substrate side firstguide layer 8 d, a ridge side second guide layer 5 d comprising undopedAlGaAs and having the same thickness and refractive index as those ofthe substrate side second guide layer 9 d, and a p type AlGaAs uppercladding layer 4 c having a refractive index nuc and a ridge at itssurface. In this fifth embodiment, the Al composition ratios of thelower cladding layer 10 c and the upper cladding layer 4 c are differentfrom each other so that the refractive index n_(Lc) of the lowercladding layer 10 c is larger than the refractive index n_(Uc) of theupper cladding layer 4 c. The lower cladding layer 10 c, the substrateside second guide layer 9 d, the substrate side first guide layer 8 d,the ridge side first guide layer 6 d, the ridge side second guide layer5 d, and the upper cladding layer 4 c respectively have fixed refractiveindices in the thickness direction x. The Al composition ratio of thesubstrate side second guide layer 9 d is smaller than the Al compositionratio of the lower cladding layer 10 c. As a result, in the firstcomposite semiconductor layer, the refractive index increases stepwisewith distance from the substrate 12 and, in the second compositesemiconductor layer, the refractive index decreases stepwise withdistance from the active layer 7 a. In FIGS. 6(a) and 6(b), the samereference numerals as those shown in FIGS. l(a) and 1(b) designate thesame or corresponding parts.

[0061] The fabrication method of the semiconductor laser according tothis fifth embodiment is identical to the fabrication method alreadydescribed for the first embodiment except that the lower cladding layer10 c, the substrate side the second guide layer 9 d, and the substrateside first guide layer 8 d are successively grown instead of the lowercladding layer 10 a, and the ridge side first guide layer 6 d, the ridgeside second guide layer 5 d, and the upper cladding layer 4 c aresuccessively grown instead of the upper cladding layer 4 a.

[0062] In this semiconductor laser, since the refractive index of thelower cladding layer 10 c is larger than the refractive index of theupper cladding layer 4 c as shown in FIG. 6(c), the refractive indexdistribution between the upper cladding layer 4 c and the lower claddinglayer 10 c in the thickness direction x is asymmetrical about the activelayer 7 a. Thereby, the center of the light intensity distribution ofthe propagated light shifts from the active layer 7 a toward thesubstrate 12, so that more light is distributed to the substrate sidethan the ridge side. As a result, the propagated light is hardlyaffected by the refractive index distribution in the width direction ydue to the presence of the ridge, and the same effects as provided bythe first embodiment are achieved. Further, in this fifth embodiment,since more light is distributed to the lower cladding layer 10 c, thewaveguide pattern of light itself is extended, so that the far fieldpattern (FFP) in the thickness direction x is narrowed. As a result, theaspect ratio, i.e., the ratio of the thickness direction FFP to thewidth direction FFP, is reduced.

[0063] Next, the effects provided by increasing the refractive index ofthe substrate side lower cladding layer 10 c will be described on thebasis of simulation results.

[0064]FIG. 7 is a graph showing the simulation results of thesemiconductor laser according to this fifth embodiment. In FIG. 7, theordinate shows the thickness t of the upper cladding layer 4 c outsidethe ridge, and the abscissa shows the ridge width W. Each of thecontinuous line and the dotted line shows the boundary between a regionwhere only the fundamental mode (0 order) is permitted and a regionwhere the primary mode (primary order) is also permitted, which boundaryis calculated from the thickness t and the ridge width W using theequivalent refractive index method. The region where only thefundamental mode is permitted is under the boundary, and the regionwhere the primary mode is also permitted is above the boundary. In thissimulation, the Al composition ratio of the upper cladding layer 4 c is0.3, the thickness of the upper cladding layer 4 c at the ridge is t+1.4μm, and the Al composition ratio of the ridge side second guide layer 5a is 0.2. The active layer 7 a has a quantum well structure in which a20 nm thick GaAs layer is put between two InGaAs layers each having anIn composition ratio of 0.15 and a thickness of 8nm. The Al compositionratio of the substrate side second guide layer 9 d is 0.2, the Alcomposition ratio of the lower cladding layer 10 c is 0.3 or 0.28, andthe thickness of the lower cladding layer 10 c is t+1.4 μm. Thesimulation result shown by the dotted line is obtained when therefractive index distribution is symmetrical about the active layer 7 a,i.e., when the Al composition ratio of the upper cladding layer 4 c isequal to that of the lower cladding layer 10 c and, therefore, thesecladding layers have the same refractive index (n_(Lc)=n_(Uc)) Thesimulation result shown by the continuous line is obtained when therefractive index distribution is asymmetrical about the active layer 7a, i.e., when the Al composition ratio of the lower cladding layer 10 cis smaller than that of the upper cladding layer 4 c and, therefore, therefractive index of the lower cladding layer 10 c is larger than that ofthe upper cladding layer 4 c (n_(Lc)>n_(Uc)).

[0065] It can be seen from FIG. 7 that, when the refractive indices ofthe upper and lower cladding layers are asymmetrical, the boundarybetween the fundamental mode and the primary mode shifts upward thanwhen the refractive indices of the upper and lower cladding layers aresymmetrical. For example, when t=0.40 μm, the boundary is positioned atthe ridge width W of about 3.5 μm in the symmetrical structure, whilethe boundary is positioned at the ridge width W of about 4.15 μm in theasymmetrical structure. Therefore, the same effects as provided by thesecond embodiment are achieved.

[0066] In this fifth embodiment, only the refractive index of the lowercladding layer 10 c is set larger than the refractive index of the uppercladding layer 4 c to make the refractive index distributionasymmetrical about the active layer 7 a. However, in addition to theasymmetrical refractive indices of the upper and lower cladding layers,the refractive indices and thicknesses of the substrate side first andsecond guide layers may be set larger than those of the ridge side upperand lower guide layers as described for the fourth embodiment of theinvention.

[0067] [Embodiment 6]

[0068]FIG. 8(a) is a cross-sectional view of a semiconductor laseraccording to a sixth embodiment of the present invention, and FIG. 8(b)is a graph showing the refractive index profile of the laser in thethickness direction x.

[0069] The semiconductor laser according to this sixth embodiment isidentical to the semiconductor laser according to the fifth embodimentexcept that n type GaAs current blocking layers 14 are disposed on theupper cladding layer 4 c, contacting both sides of the ridge of thecladding layer 4 c, thereby to realize a gain waveguide type buriedridge semiconductor laser. Further, insulating films 2 a are disposed onthe current blocking layers 14 and on the contact layer 3 except aportion of the top of the contact layer 3, and a p side electrode 1 a isdisposed contacting the contact layer 3 at the top. This semiconductorlaser is fabricated as follows. That is, in the fabrication process ofthe semiconductor laser according to the fifth embodiment, after theridge is formed by selective etching with an insulating film as a mask,the current blocking layers 14 are selectively grown using the mask.

[0070] The buried ridge type semiconductor laser according to this sixthembodiment provides the same effects as provided by the fifth embodimentof the invention.

[0071] In this sixth embodiment, emphasis has been placed on a buriedridge type semiconductor laser obtained by burying the ridge structureof the semiconductor laser according to the fifth embodiment with thecurrent blocking layers 14. However, a buried ridge type semiconductorlaser obtained by burying the ridge structure of the semiconductor laseraccording to any of the first to fourth embodiments with the currentblocking layers 14 is also within the scope of the invention.

[0072] Furthermore, although in this sixth embodiment the currentblocking layers 14 comprise n type GaAs, current blocking layerscomprising other materials may be adopted with the same effects asdescribed above. For example, n type AlGaAs current blocking layershaving an Al composition ratio higher than that of the upper claddinglayer may be adopted to provide a refractive index waveguide typesemiconductor laser.

[0073] In the first to sixth embodiments of the invention, therefractive index distribution viewed from the active layer is madeasymmetrical by changing the thickness or the refractive index of atleast one of the lower cladding layer, the substrate side first andsecond guide layers, the ridge side first and second guide layers, andthe upper cladding layer, which are constituents of the firstsemiconductor laser between the active layer and the substrate and thesecond semiconductor layer disposed on the active layer and having aridge at its surface. However, in the present invention, the sameeffects as those provided by the first to sixth embodiments are achievedas long as the refractive indices of the first semiconductor layer andthe second semiconductor layer are respectively fixed in the thicknessdirection or are gradually decreased with distance from the activelayer, and the refractive index distribution in the thickness directionis asymmetrical about the active layer so that the center of the lightintensity distribution shifts from the active layer toward thesubstrate.

[0074] While a refractive index waveguide type semiconductor laser isdescribed in the first to fifth embodiment and a gain waveguide typesemiconductor laser is described in the sixth embodiment, the sameeffects are achieved in the present invention regardless of the type ofthe semiconductor laser.

[0075] Furthermore, while in the first to sixth embodiments of theinvention AlGaAs is employed as the material of the lower claddinglayer, the substrate side first and second guide layers, the ridge sidefirst and second guide layers, and the upper cladding layer, othermaterials may be employed with the same effects as provided by theseembodiments.

[0076] Moreover, while in the first to sixth embodiments GaAs isemployed as the material of the substrate, other materials may beemployed with the same effects as provided by these embodiments.

What is claimed is:
 1. A semiconductor laser including: a semiconductorsubstrate of a first conductivity type and having a front surface; afirst semiconductor layer disposed on the front surface of saidsemiconductor substrate and having a refractive index that increaseswith distance from said semiconductor substrate; an active layerdisposed on said first semiconductor layer; and a second semiconductorlayer disposed on said active layer, having a refractive index thatdecreases with distance from said active layer, and having a ridge;wherein the refractive index distribution between said ridge and saidsubstrate is asymmetrical about said active layer so that the center ofthe light intensity distribution shifts from the active layer towardsaid substrate, in the direction perpendicular to the front surface ofsaid substrate.
 2. The semiconductor laser of claim 1 wherein: saidfirst semiconductor layer comprises a lower cladding layer of the firstconductivity type and having a refractive index, and a substrate sideguide layer disposed on said lower cladding layer and having a thicknessand a refractive index; said second semiconductor layer comprises aridge side guide layer having a thickness and a refractive index, and anupper cladding layer of a second conductivity type, opposite the firstconductivity type, disposed on said ridge side guide layer and having arefractive index; and at least one of the thickness and the refractiveindex of said substrate side guide layer is larger than that of saidridge side guide layer.
 3. The semiconductor laser of claim 2 whereinthe refractive index of said lower cladding layer is larger than that ofsaid upper cladding layer.
 4. The semiconductor laser of claim 1wherein: said first semiconductor layer includes a lower cladding layerof the first conductivity type and has a thickness and a refractiveindex; said second semiconductor layer includes an upper cladding layerof a second conductivity type, opposite the first conductivity type, andhas a thickness and a refractive index; and at least one of thethickness and the refractive index of said lower cladding layer islarger than that of said upper cladding layer.
 5. The semiconductorlaser of claim 4 wherein: the refractive index of said lower claddinglayer continuously increases toward said active layer; and therefractive index of said upper cladding layer continuously decreaseswith distance from said active layer.